exp 3 oscilloscope and waveform generator (2012)

7/28/2019 Exp 3 Oscilloscope and Waveform Generator (2012)

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EEE 125 LAB 3

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Universiti Sains MalaysiaElectrical & Electronic Engineering DepartmentBasic Circuits Laboratory EEE125

Oscilloscope, Probes and Waveform GeneratorOBJECTIVE

To become familiar with using an oscilloscope and a built-in waveform generator.

To calculate, obtain, and measure the time varying signals, amplitude and duration(period) of various voltage signals.

To explore time domain and frequency domain representation of signals.

INTRODUCTION

Digital Oscilloscope

The oscilloscope is the most important instrument available to the practicing technician orengineer. It permits the visual display of a voltage signal that can reveal a range of informationregarding the operating characteristics of a circuit or system that is not available with a standardmultimeter. At first glance the instrument may appear complex and difficult to master. Be assured,however, that once the function of each section of the oscilloscope is explained and understoodand the system is used throughout a set of experiments, your expertise with this important toolwill develop quite rapidly.

Now that we have acquired a basic understanding of how the oscilloscope works, we canproceed to explore some of the most common tasks which can be accomplished with it. Theoscilloscope which we will be using in the lab is the InfiniiVision DSO-X 2002A Oscilloscopes , atwo channel (input) scope. Aside from the display itself, the oscilloscope consists of variouscontrol panels which give the user complete control of how the scope processes and displays its

inputs. In the section that follows we will describe each of the control panels and outline thefunctionality of each of the settings found therein.

The oscilloscope is basically a graph displaying apparatus and it represents an electrical signalsuch as voltage as a function of time. Our oscilloscope has two channels and the ability toperform the basic functions of Triggering and scaling both in the vertical and the horizontal axes.In the laboratory environment one usually finds oscilloscopes that perform a variety of specializedfunctions including various mathematical operations on signals. The features of our scopecorresponds to the most basic and important characteristics of these instruments.


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The Front Panel Controls

On the front panel, keyrefers to any key (button) you can press. Softkeyspecifically refers to thesix keys that are directly below the display. The legend for these keys is directly above them, onthe display. Their functions change as you navigate through the oscilloscope's menus. For thefollowing figure, refer to the numbered descriptions that follows.

Figure 1: InfiniiVision DSO-X 2002A Oscilloscope

The Oscilloscope Display

The oscilloscope display contains acquired waveforms, setup information, measurement results,and the softkey definitions.

Figure 2: Oscilloscope Display


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Frequency Measurement in Oscilloscope

The frequency of an unknown signal may be calculated from the oscilloscope very easily. Theperiod of the waveform is the product of the distance along the x-axis covered by one cycle andthe horizontal sweep setting. As an example, a sine-wave generator is set to 1000 Hz with thevoltage applied to the oscilloscope vertical. One cycle covers 9.95 cm, with a sweep speed (or

known as time/div) of 100 sec/cm. The period is T = (9.95)*(100X10

-6

)sec. The measuredfrequency is f= 1/T = 1005 Hz.

Figure 2: Frequency measurement

EQUIPMENTS

Resistors 1M X 1, 2M X 1

Breadboard

Digital multimeter

DSO-X 2002A Digital Oscilloscopes

USB data drive (bring this during lab session)

Crocodile clips, BNC Connectors, Connection wires (single core)


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Experiment: Part IOscilloscope Probes: Theory and Practice

1. Measuring an electrical signal inevitably affects that signal. This applies to allmeasurements, including the display of an oscilloscope waveform. Affecting the signalcannot be totally eliminated, but it can be minimized sufficiently that the effect isunimportant.

2. Then the measured result is a sufficiently accurate representation of the real signal. It istherefore critical for the measurement engineer to understand the effect of the instrumenton the signal. A x10 scope probe (Figure 1a) is useful in several applications:

a) To reduce loading effecton the circuit under testb) To compensate for the effect of test cable capacitancec) To permit the measurement of large voltages

Figure 1a: Oscilloscope Probe

3. The better an oscilloscope or voltmeter is, the smaller the current it needs to flow throughit of a given voltage - i.e. the higher its input resistance. Since it has a very large inputresistance, an oscilloscope can only be used to measure voltages and time.

4. To better understand the loading effect of oscilloscope or voltmeter, consider the circuit ofFigure 1b, one volt in series with a resistance of 1M. (That is, the Thevenin Equivalentis a circuit with an open-circuit voltage of 1 volt, and an internal resistance of 1M.)Suppose an ideal voltmeter/oscilloscope, which presents an open circuit to themeasurement circuit, is used to measure the voltage. There is no current through an idealvoltmeter/oscilloscope, so there is no voltage drop in the resistor.


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Figure 1b: Ideal voltmeter/oscilloscope

5. Expect the voltage between points A and B.VAB = __________ V (expected)

6. Using digital voltmeter, measure the voltage across points A and B.

VAB = __________ V (measured)

7. Now consider Figure 1c. The same measurement is attempted with a digital voltmeterthat presents a load of say, 2M to the circuit. Then the 1M internal resistance and the2M voltmeter resistance Rmeterform a voltage divider.

Figure 1c: Non-ideal voltmeter/oscilloscope

8. Expect the voltage between points A and B.VAB = __________ V (expected)

9. Using digital voltmeter, measure the voltage across points A and B.VAB = __________ V (measured)

10. Explain the differences between ideal and non-ideal voltmeter/oscilloscope.


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Experiment: Part IICalibrating the oscilloscope probes, probe compensation and frequency measurement

1. Press the power switch. The power switch is located on the lower left corner of the frontpanel. The oscilloscope will perform a self- test and will be operational in a few seconds.

2. The first signal to input to the oscilloscope is the Demo 2 Probe Comp signal (see Figure1 no.18). This signal is used for compensating probes.

3. Connect an oscilloscope probe from channel 1 to the Demo 2(Probe Comp) terminal onthe front panel.Connect the probe's ground lead (reference lead) to the ground terminal(next to the Demo 2terminal).

4. Use [Auto Scale] to automatically configure the oscilloscope to best display the inputsignals. Press [Auto Scale]. You should see a waveform on the oscilloscope's display acompensated signal similar to Figure 2a.

Figure 2a: Calibrated and compensated signal

5. Otherwise you will get uncompensated signal like in Figure 2b. The compensationcapacitance of the probes must be adjusted to suit the input capacitance of theoscilloscope. This requires adjusting the compensation each time if the probe is moved toa different scope. It is driven with a purely square wave signal. To compensate the probe,the compensation capacitance may typically be found as a screwdriver adjustment in thebody of the probe. By adjusting the screw you may obtain the compensated signal.

Figure 2b: Uncompensated signal


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Experiment: Part IVWaveforms generator

1. A waveform generator is built into the oscilloscope (comes together). The waveformgenerator gives you an easy way to provide input signals when testing circuitry with theoscilloscope. Waveform generator settings can be saved and recalled with oscilloscopesetups.

2. To access the Waveform Generator Menu and enable or disable the waveform generatoroutput on the front panel Gen Out BNC, press the [Wave Gen ] key. When waveformgenerator output is enabled, the [Wave Gen] key is illuminated. When waveformgenerator output is disabled, the [Wave Gen] key is off.

3. The waveform generator output is always disabled when the instrument is first turned on.The waveform generator output is automatically disabled if excessive voltage is appliedto the Gen Out BNC.

4. In the Waveform Generator Menu, press the Waveformsoftkey and turn the Entry knobto select the waveform type.

Figure 4a: Sample of waveform generators output

5. Depending on the selected waveform type, use the remaining softkeys and the Entryknob to set the waveform's characteristics.

6. Pressing a signal parameter softkey can open a menu for selecting the type ofadjustment. For example, you can choose to enter amplitude and offset values, or youcan choose to enter high- level and low- level values. Or, you can choose to enterfrequency values or period values. Keep pressing the softkey to select the type ofadjustment. Turn the Entry knob to adjust the value.

7. Notice that you can select between coarse and fine adjustments for frequency, period,and width. Also, pushing the Entry knob is a quick way to toggle between coarse and fineadjustments.

8. The Settings softkey opens the Waveform Generator Settings Menu which lets you makeother settings related to the waveform generator. For example to restore waveformgenerator defaults, press the Settings softkey. Then press the Default Wave Gen softkey.The waveform generator factory default settings (1 kHz sine wave, 500 mVpp, 0 V offset,High- Z output load) are restored.


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9. For our first experiment with the scope we will measure the various signals from thewaveform generator. Connect an oscilloscope probe from channel 1 to the Gen Out BNCoutput using any extra BNC cable obtained from lab technician.

10. Start by setting the waveform generator to generate a sinusoidal signal with a peak topeak voltage and frequency of 5 Vp-p and 10 kHz respectively. Measure that signal onchannel 1 of your scope. Press [Auto Scale] and adjust the volt/div and time/div until yousee the signal. Draw your output signal in Figure 4b.

Figure 4b: 5 Vp-p, 10 kHz sinusoidal waveform

11. Set the volt/div to 1V/div and time/div to 25 us/div on the scope. Set the offset voltage to0 V. Determine the number of divisions of x-axis and y-axis.

Number of divisions, x-axis = __________ at 25us/divNumber of divisions, y-axis = __________ at 1V/div

12. A sinusoidal signal can also be represented by a mathematical function. Set thewaveform generator to generate a sinusoidal signal with a corresponding function. Drawyour output signal in Figure 4c. The function is V(t)= 2.5 sin 314t + 0.

Figure 4c: V(t)=2.5 sin 314t + 0 sinusoidal waveform


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13. Determine the number of divisions of x-axis and y-axis. Record the volt/div and time/divfrom the display, thus measure the period, frequency and peak to peak voltage as well asAC RMS-FS voltage of the calibrated signal using automated measurement.

Number of divisions, x-axis = __________Number of divisions, y-axis = __________Volt/div = __________ VTime/div = __________ sPeriod = __________ s (measured)Frequency, f = __________ Hz (measured)Pk-Pk voltage, Vp-p = __________ V (measured)AC RMS-FS voltage, Vac,rms = __________ V (measured)

14. Let say the following signal is a PWM output obtained from an experiment. Determine thetype of the signal, the number of divisions of x-axis and y-axis. Record the volt/div andtime/div from the display, thus determine the period, frequency, peak to peak voltage andduty cycle of the signal.

Figure 4d: PWM output signal

Type of signal = __________Number of divisions, x-axis = __________Number of divisions, y-axis = __________Volt/div = __________ VTime/div = __________ sPeriod = __________ s (calculated)Frequency, f = __________ Hz (calculated)Pk-Pk voltage, Vp-p = __________ V (calculated)Duty cycle, k = __________ % (calculated)


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REVIEW QUESTIONS

1. List the uses of oscilloscope and waveform generator.

2. List the waveforms that can be generated via waveform generator.

3. Determine the period of 5 Vp-p, 1000 Hz sinusoidal waveform in milliseconds.Period = __________ ms

If the horizontal sensitivity of the scope is set to 2 ms/div, calculate the number of

horizontal divisions required to properly display one full cycle of the 1000 Hz signal.

Number of divisions = __________

If the vertical sensitivity of the scope is set to 0.2 V/div, calculate the number of vertical

divisions required to properly display full cycle of the signal. Can the oscilloscope do this?

Number of divisions = __________

TURN IN ONE REPORT PER GROUP AT THE END OF YOUR LAB SESSION.THERE IS NO TAKE HOME REPORT.

exp 3 oscilloscope and waveform generator (2012)

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